1. Field of the Invention
The present invention relates to an electrodeless high intensity discharge lamp and more particularly pertains to protecting arc tubes by locating a film of excess liquid metal halide in those areas of the arc tube that are most subject to arc erosion, the stabilization being achieved by a mechanically rough surface or a layer of metal oxide powder.
2. Description of the Prior Art
High-pressure, electrodeless, inductively driven gas discharge lamps offer attractive combinations of high efficacy and good color rendition. In order to be economically competitive, such lamps must operate for many thousands of hours without substantial degradation of light output. A major problem with achieving long lamp life is the erosion of those areas of the arc tube that are close to the intense discharge.
In particular, erosion has been observed during life tests conducted upon inductively driven, electrodeless gas discharge lamps. The tested lamps use quartz arc tubes of cylindrical shape with rounded corners. The temperature of the arc tubes ranges from 850 C to 1000 C. The arc tubes are dosed with an inert buffer gas and metal halides, such as sodium iodide and cerium iodide creating a fill or "dose". The metal halide pressure in an operating arc tube is controlled by the temperature of a liquid reservoir of excess metal halide. This reservoir forms at the coolest portions of the inside surface of the arc tube.
It has been observed that after prolonged operation, a damage zone appears on the inside surface of the arc tube. This damage zone is in the form of a ring or annular region that is located along the periphery of the cylindrical arc tube. This is also the region where the intense arc is pressed against the tube surface by the induced radio frequency (RF) field.
The exact mechanisms that lead to the arc tube damage have not yet been fully clarified. It is believed that under the intense ion bombardment and radiation from the arc, chemical reactions occur that lead to arc tube degradation. For instance, sodium iodide is dissociated by the arc into positive sodium ions and negative iodine ions. The positive sodium ions are driven towards the wall by the electric field of the arc. If even a small fraction of these ions do not recombine with iodine before reaching the wall, then the sodium can attack the quartz wall chemically. Other metal halides in the arc tube dose, such as rare earth iodides, may produce arc tube damage in a similar manner. The net result is a loss of the metallic constituents, such as sodium, leading to degradation of the light output, and a buildup of free halide, such as iodine, that leads to arc instability and eventual arc extinction.
There is substantial literature on the loss of sodium in high-pressure metal halide lamps. Early work is reviewed by John F. Waymouth in his book on Electric Discharge Lamps (MIT Press, 1971, pp 266-277). E. Fischer contributed a paper on "Formation of Free Iodine in Metal Halide Lamps" to the 1988 Symposium in High Temperature Lamp Chemistry. The sodium loss is attributed to diffusion of sodium through the arc tube wall as well as to reactions within the arc tube.
A method of using a protective metal halide film in high-pressure, electrodeless discharge lamps is described in U.S. Pat. No. 5,032,757, issued Jul. 16, 1991, to Witting. In that patent, the portion of the arc tube wall which is nearest the plasma arc discharge is maintained at a lower temperature than the remainder of the arc tube, so that a condensate of metal halide forms a protective layer thereon. The Witting patent discloses an electrodeless high intensity discharge lamp having an excitation coil disposed about an arc tube which includes thermal apparatus for ensuring that a metal halide condensate forms a protective film on the portion of the arc tube which is nearest the plasma arc discharge during lamp operation. For a short, cylindrical arc tube, the thermal apparatus comprises a heat shield situated on the top and/or bottom thereof. In one embodiment, the bottom of the arc tube is concave to ensure that the condensate does not collect on the bottom of the arc tube. The excitation coil may be situated sufficiently close to the arc tube to ensure that enough heat is removed from the side wall of the arc tube to a heal sink so that the protective metal halide film forms on the inner surface of the arc tube wall. An outer glass envelope is preferably situated between the arc tube and the excitation coil, which envelope also functions to remove heat from the arc tube side wall.
A practical problem has been observed in the use of a lamp having at least some of the above-described features. For example, on new arc tubes with smooth inside surfaces, the liquid dose forms droplets that are large enough to move downwards periodically to hotter portions of the arc tube under the force of gravity. From there the dose evaporates and re-condenses on cooler surfaces. This periodic movement tends to expose bare arc tube surfaces to degradation by the nearby arc. It is also accompanied by very undesirable changes in the position of the arc itself. In other words, the instability in the dose location causes arc instabilities that are not acceptable in a commercial light source. Thus, it would be desirable to provide such a lamp which overcomes the above and other disadvantages of the prior art.